Since the development of synthetic aperture, attempts have been made to reduce the size, weight, power consumption, and cost of these systems. Basically, a target crossing a radar beam produces a chirp signal at the output of a detector. The chirp signal is then processed electronically in a matched filter in real time or is recorded and then reproduced at a later time in an optical matched filter. The real time operation of the electronic processor is obtained at the expense of large size, weight, and power consumption of the processor while the small size, weight, and power consumption of the optical processor is obtained at the expense of an annoying delay of obtaining images. Examples of conventional synthetic aperture can be found in the paper by L. Cutrona et al., "On the Application of Coherent Optical Processing Techniques to Synthetic Aperture" appearing in the August, 1966 issue of Proc. IEEE, and in the paper by W. Brown and L. Porcello, "An Introduction to Synthetic Aperture Radar" appearing in the September, 1969 issue of IEEE Spectrum. While these systems first record detected signals and then reproduce images in a time consuming two step process, my U.S. Pat. Nos. 3,691,557 System for Identifying Objects Using an Encoding Array for each Object, 3,790,939 Real Time Synthetic Radar, 3,798,644 Vector Velocity System, and 3,858,203 Synthetic Radiometer and System for Identifying Objects are examples of systems which process their signals electronically in real time.
While all the known apparatus and methods of the prior art have the ability to form synthetic apertures, they have significant inherent disadvantages. For example, systems using optical processing while significantly reducing the size, weight, and power consumption of processors and increasing the data capacity for two dimensional imaging do so at the expense of a two step recording and then reproducing cycle which often is time consuming in many applications. On the other hand, systems using electronic processing while operating in real time do so at the expense of having large size, weight, and power consumption of their processors and having a reduced data capacity. Because of these disadvantages, the conventional synthetic aperture has found limited use, mainly in military surveillance, reconnaisance, and ground mapping applications and has found no commercial application whatsoever. It is desired to provide a synthetic aperture apparatus and method that overcomes these disadvantages so that synthetic aperture will have a widespread and commercial use. More specifically, it is desirable to provide a synthetic aperture apparatus and method that has a real time operation yet has a small size, weight, power consumption, and cost, and high capacity, i.e., combining the best features of prior art systems while eliminating their disadvantages.
From the foregoing it will be appreciated that the invention is concerned with producing a synthetic aperture for objects crossing the receiving beam. The generation of a synthetic aperture in the prior art and in the system of the invention is based on the matched filtering of signals received from objects crossing a receiving beam. However, whereas the prior art first detects signals and then performs matched filtering, the invention first performs matched filtering and then detects signals, such matched filtering being provided by the delay means of the invention acting as a predetection matched filter of signals from objects. Importantly, the invention provides capabilities over the prior art in the measurement of broadband targets.
Matched filtering in the prior art and in the system of the invention requires the spectral coherency between transmissions and receptions of signals. However, the system of the invention also enables the measurement of broadband targets by cosine modulating signals from targets, such cosine modulation being provided by the delay means of the invention acting as a beamsplitter-recombiner of signals from objects.
Therefore, it is an object of this invention to provide apparatus and method for a synthetic aperture that operates in real time.
It is also an object of this invention to provide apparatus and method for a synthetic aperture having small size, weight, power consumption, and high capacity.
It is another object of this invention to provide apparatus and method for a synthetic aperture which detects already synchronized or focused signals and thereby eliminates the further post-detection processing of signals indicated by the prior art.
It is another object of this invention to provide apparatus and method for a focused beam synthetic aperture which operates in either one of the microwave or optical portions of the electromagnetic spectrum.
It is a further object of this invention to provide apparatus and method for a focused beam synthetic aperture which is capable of readily changing its focus.
It is yet another object of this invention to provide apparatus and method which is capable of operating passively using coherent or noncoherent objects as sources.